On Mon, Mar 10, 2014 at 8:59 AM, Derek McLoughlin < derek.mcloughlin@gmail.com> wrote:
ghci> distance (x, x) $ toVector (y, y) 12.727922061357855
I don't understand why this doesn't work:
ghci> let z = toVector (y, y) interactive>:32:9: No instance for (Vectorizable (Double, Double) v0) arising from a use of `toVector' The type variable `v0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there is a potential instance available: instance Vectorizable (Double, Double) (Double, Double) -- Defined at vector.hs:13:10 Possible fix: add an instance declaration for (Vectorizable (Double, Double) v0) In the expression: toVector (y, y) In an equation for `z': z = toVector (y, y)
It seems odd that "toVector" works when used as an argument to "distance" but not when used in a let expression.
Can anyone explain?
When you use them together, Haskell can infer the correct type for `v0` from the inferred type of `distance` by applying defaulting: the type of `sqrt` introduces a constraint that is subject to defaulting. If you separate them, it can no longer determine a concrete type for your `let`, as multiparameter type classes are not subject to defaulting (and can't be). Also, Haskell cannot conclude from the existence of a single possible instance that it should use that instance: the meaning of your expression would then change if you imported a module which defined a new instance. The requirement for a concrete type comes from the monomorphism restriction, which is applied to your `let` because `z` has no parameters. http://www.haskell.org/haskellwiki/Monomorphism_restriction Many people turn off the monomorphism restriction in ghci to avoid this kind of issue. :set -XNoMonomoprhismRestriction which you can put in ~/.ghci so that it is the default for new ghci sessions. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net